The invention relates to a thermal protection relay designed to protect an electrical apparatus, comprising means for measuring a current absorbed by the apparatus, means for determining a value representative of the thermal image of the apparatus according to the value of the measured current, to a previous value of the thermal image and to at least one preset time constant, means for determining at least one protection threshold, means for comparing the value representative of the thermal image with the protection threshold and means for producing an alarm and/or tripping signal when the value representative of the thermal image exceeds the protection threshold.
Such a relay 1 can be used for protection of a motor 2 (FIG. 1), a transformer, an electrical line or a capacitor bank. As represented in FIG. 1, current sensors 3 supply the relay 1 with signals representative of the current absorbed by the motor 2. The relay 1 computes, from the measured currents, a value representative of the thermal image of the motor 2, i.e. representative of the heating of the motor. In the event of an excessively large heating due for example to an overload and exceeding respectively an alarm threshold Esa or a tripping threshold Esd, the relay 1 supplies an alarm signal or a tripping signal. The tripping signal causes opening of contacts 4 and interruption of the power supply of the motor 2.
In conventional manner, a value Ek of the thermal image is computed, at a time t, according to the equation:                               E          k                =                              E                          k              -              1                                +                                                    (                                                      I                    eq                                                        I                    b                                                  )                            2                        ·                                          Δ                ⁢                                  xe2x80x83                                ⁢                t                            τ                                -                                    E                              k                -                1                                      ·                                          Δ                ⁢                                  xe2x80x83                                ⁢                t                            τ                                                          (        1        )            
in which:
Ek-1 is the previous value of the thermal image, computed at the time (t-xcex94t).
Ieq is an equivalent current, representative of the current absorbed by the motor and determined from the measured currents.
Ib is the base current of the apparatus, i.e. the current absorbed by the apparatus in operation under nominal operating conditions.
xcfx84 is the time constant of the apparatus to be protected. Conventionally, the time constant xcfx84 can take two distinct values, a heating time constant xcfx841 when the motor is running, and a cooling time constant xcfx842, supplied by the manufacturer, when the motor is not running.
In known manner, the equivalent current Ieq is computed from the measured currents according to the equation:
Ieq2=I2+KIinv2xe2x80x83xe2x80x83(2)
in which:
I is the largest of the measured rms currents flowing in phase conductors supplying the apparatus
Iinv is the measured inverse current
K is an adjustable coefficient.
As represented in FIG. 2, cold and hot time/current tripping curves of the relay, respectively Cf1 and Cc1 (in broken lines), can thus be defined.
The cold tripping curve Cf1 defines the tripping time of the protection relay from zero heating, according to the equation:                               t          τ                =                  log          ⁢                      xe2x80x83                    ⁢                                                    (                                  Ieq                                      I                    b                                                  )                            2                                                                        (                                      Ieq                                          I                      b                                                        )                                2                            -                              E                sd                                                                        (        3        )            
in which Esd is the tripping threshold.
The hot tripping curve Cc1 defines the tripping time of the protection relay from a nominal heating level E=1, according to the equation:                               t          τ                =                  log          ⁢                      xe2x80x83                    ⁢                      (                                                                                (                                          Ieq                                              I                        b                                                              )                                    2                                -                1                                                                                  (                                          Ieq                                              I                        b                                                              )                                    2                                -                                  E                  sd                                                      )                                              (        4        )            
For a preset current greater than a maximum current in steady-state operating conditions, the tripping time from a cold state of the motor to be protected is greater than that obtained from a hot state of the motor.
As an example, in FIG. 2, for a current Ieq/Ib=2, the tripping time of the relay is 665 s when cold (point A1) and 70 s when hot (point A2).
Manufacturers generally provide experimental hot and cold thermal resistance curves of the apparatus. In FIG. 2, the curves of the motor when hot Cc1m and cold Cf1m (in unbroken lines) are shifted upwards with respect to the associated curves Cc1 and Cf1 of the relay. Protection of the corresponding motor is therefore performed correctly by the relay.
It does however happen that the hot and cold thermal resistance curves of the motor (Cc2m and Cf2m) are much closer than those of the relay (Cc2 and Cf2), as represented in FIG. 3. In the example represented in FIG. 3, for a current Ieq/Ib=2, the hot thermal resistance time of the relay is 250 s (point A3), as previously lower than the corresponding thermal resistance time of the motor. However, the cold tripping time of the relay is 620 s (point A4), greater than the cold thermal resistance time (point A5) of the motor. The motor is therefore not protected correctly when it is subjected to an overload from a cold state, although tripping from a hot state remains assured within the necessary time.
By lowering the heating time constant xcfx841 of the relay, the two tripping curves of the relay can be shifted downwards. The new hot tripping curve Cc3 and cold tripping curve Cf3 thus obtained are both situated below the associated curves of the motor (Cc2m and Cf2m). The new cold thermal resistance time (point A6) is then lower than the cold thermal resistance time of the motor (point A5). The same is true for the hot thermal resistance time (points A7 and A3).
However, lowering the hot tripping curve of the relay can give rise to problems on start-up. The hot tripping curve Cc3 of the relay can in fact cross the motor start-up curve, as represented in FIG. 3. In FIG. 3, two start-up curves Cd1 and Cd2 are represented. Each of these curves represents the current value versus time, on start-up of the motor, respectively for a start-up with rated voltage Un (Cd1) and for a start-up with a voltage of 0.9 Un (Cd2).
The hot tripping curve Cc3 crosses the start-up curve Cd1 at the point A8 and the start-up curve Cd2 at the point A9. Thus, in case of a hot start-up, the current value is such that it immediately causes tripping, thus preventing any hot start-up.
The object of the invention is to overcome these drawbacks and to provide a good thermal protection of the apparatus in all circumstances.
According to the invention, this object is achieved by the fact that a non-zero initial thermal image is taken into account for determining the thermal image, the value of the initial thermal image being determined from experimental thermal resistance curves of the apparatus.
According to a development of the invention, the protection threshold being a tripping threshold, the cold tripping time t of the relay is given by the equation:                               t          τ                =                  log          ⁢                      xe2x80x83                    ⁢                      (                                                                                (                                                                  I                        eq                                                                    I                        b                                                              )                                    2                                -                                  E                  s0                                                                                                  (                                                                  I                        eq                                                                    I                        b                                                              )                                    2                                -                                  E                  sd                                                      )                                              (        5        )            
in which:
xcfx84 is the time constant of the relay
Ib is the base current of the apparatus
Ieq is an equivalent current representative of the measured current
Es0 is the initial thermal image
Esd is the tripping threshold.
The initial thermal image Es0 is preferably determined according to the equation:                               E          s0                =                                            (                                                I                  r                                                  I                  b                                            )                        2                    -                                    ⅇ                                                t                  r                                                  τ                  1                                                      ·                          [                                                                    (                                                                  I                        r                                                                    I                        b                                                              )                                    2                                -                                  E                  sd                                            ]                                                          (        6        )            
in which:
Ib is the base current of the apparatus
Ir is a preset setting current
tr is a required cold tripping time associated to the setting current Ir
xcfx841 is a heating time constant, determined from an experimental hot thermal resistance curve of the apparatus.
The relay can, in addition, comprise means for measuring the ambient temperature and means for correcting the value representative of the thermal image according to the ambient temperature measured.
According to another development of the invention, in the case of a motor, the time constants, alarm and/or tripping threshold and initial thermal image constituting a set of parameters of the relay, the relay comprises means for comparing the measured current and a current threshold representative of a blocked state of the motor rotor, and means for selecting a first set of parameters when the measured current is lower than the current threshold and for selecting a second set of parameters when the measured current is higher than the current threshold.